1. Field of the Invention
The present invention relates to an optical bus for propagating signal light and to a signal processor for carrying out signal processing including the transmission and reception of a signal using the optical bus.
2. Related Art
The number of circuit functions of a circuit board (daughter board) used in a data processing system has been greatly increasing due to the development of a very-large-scale integrated circuit (VLSI). Since the number of signals connected to each circuit board grows along with an increase in the number of circuit functions, parallel architecture which requires large numbers of connectors and connection lines is now being employed in a data bus board (mother board) for connecting the circuit boards (daughter boards) using a bus structure. Although the operation speed of a parallel bus has been improved by promoting the parallel architecture by using the multilayer structure of connection lines and reducing the width of each connection line, the processing speed of a system may be limited by the operation speed of the parallel bus due to a signal delay caused by capacity between connection lines and connection line resistance. An electromagnetic interference (EMI) problem caused by an increase in the density of connection lines of a parallel bus is a great restriction on the improvement of the operation speed of the system.
In recent years, optical communication has been making quick progress and demand for an optical branching unit which is a key device for communication has been increasing each year. However, the biggest problem of optical communication is that the cost of this key device and its packaging cost are extremely high. The cause of boosting the packaging cost is that extremely high optical positioning accuracy is required for the device for optical communication.
To solve the above problem and improve the operation speed of a parallel bus, use of intra-system optical connection technology called "optical interconnection" has been studied. As is outlined in Teiji Uchida, "the Lecture Meeting of Circuit Packaging Technology", 15C01, pp.201-202 and H. Tomimuro et al., "Packaging Technology for Optical Interconnects", IEEE Tokyo No. 33, pp.81-86, 1994, various optical interconnection technologies have been proposed according to the configuration of a system.
Out of the optical interconnection technologies which have been proposed heretofore, Japanese Published Unexamined Patent Application No. Hei 2-41042 discloses the application of an optical data transmission system which uses high-speed and high-sensitivity light emitting and light receiving devices to a data bus. This application proposes a serial optical data bus for loop transmission between circuit boards in which light emitting and light receiving devices are arranged on both front and rear sides of each circuit board and the light emitting and light receiving devices on adjacent circuit boards incorporated in a system frame are spatially interconnected by light. In this system, signal light transmitted from one circuit board is converted into an electric signal by a circuit board adjacent to the circuit board and further converted into an optical signal by the circuit board, and the converted optical signal is then transmitted to the next adjacent circuit board. Thus, the circuit boards are arranged in series and opto-electric and electro-optical conversions are repeated on each circuit board so that signal light is transmitted to all the circuit boards incorporated in the system frame. Therefore, the signal transmission speed depends on and is restricted by the opto-electric and electric-optical conversion speeds of the light emitting and light receiving devices arranged on each circuit board. Since optical interconnection of light emitting and light receiving devices arranged on each circuit board with a free space interposed therebetween is used for data transmission between the circuit boards, optical positioning of the light emitting/light receiving devices arranged on both front and rear sides of adjacent circuit boards must be carried out to optically interconnect all the circuit boards. Further, as the circuit boards are interconnected with a free space interposed therebetween, a data transmission failure might be caused by the occurrence of an interference (crosstalk) between adjacent optical data transmission lines. Further, a data transmission failure might also be caused by the diffusion of signal light by an environment within the system frame, for example, dust. Since the circuit boards are arranged in series, when any one of the boards is removed, interconnection is disconnected and an extra circuit board is required to compensate for the removed board. In other words, the circuit boards cannot be removed freely and the number of circuit boards is fixed.
Other data transmission technology between circuit boards is disclosed in Japanese Published Unexamined Patent Application No. Sho 61-196210. The technology disclosed herein is a system for optically connecting circuit boards through an optical path constituted by diffraction gratings and reflection elements arranged on the front surface of a plate having two parallel surfaces. However, this system has various problems: since light emitted from one point can be connected to only one fixed point, all the circuit boards cannot be interconnected unlike an electric bus; a data transmission failure might be caused by an interference (crosstalk) between adjacent optical data transmission lines due to a positional difference between optical elements as positioning is very difficult; and a circuit board cannot be removed freely because information on connection between circuit boards is determined by diffraction gratings and reflection elements arranged on the front surface of the plate, resulting in low extendability.
Japanese Published Unexamined Patent Application Nos. Hei 8-15539 and Hei 8-5852 propose optical branching units all of which require interconnection (packaging) having high positioning accuracy. For instance, in Japanese Published Unexamined Patent Application No. Hei 8-5852, when there is a positional difference of 4 .mu.m, signal light attenuates by -4 dB. Thus, as requirement for positioning accuracy is extremely high, packaging costs become huge, thereby retarding the popularization of the optical branching unit.
As means for solving these problems, there has been proposed a system in which circuit boards are arranged around a transparent optical transmission plate and optical signals are transmitted and received through the optical transmission plate, that is, using the optical transmission plate as an optical bus (refer to "Optics", Vol. 24, No. 9, September, 1995, pp. 574(50)-580 (56), "Board-to-Board Optical Interconnection Using Multiple Wavelengths and Stacked High Reflection Plates", C. An, T. Minemoto and "Optics", Vol.25, No. 6, 1996, pp. 337(43)-344 (50), "Architecture of Fast Fourier Transform Special-Purpose Computer Using Wavelength-Multiplexed Optical Interconnection", C. An, T. Minemoto).
However, when the optical transmission plate (optical bus) is used, light incident upon the optical transmission plate spreads to a wide range and not only to a signal light output portion for picking up the light. Therefore, optical energy use efficiency is low and there remains a problem to be solved to increase speed and reduce power consumption. Therefore, in the above system, signal light is enclosed inside by making highly reflective end surface portions excluding the signal light input portion and signal light output portion of the optical transmission plate. However, the optical energy use efficiency is improved to a certain degree but has its limits.